The present invention relates to radiation monitoring and, more particularly, to devices for calibrating radiation dosimeters.
The danger of exposure to radioactive sources has been known since the early days of this century. Madame Curie, joint discoverer with her husband of polonium and radium, was severely affected by the harmful radiation to which her research exposed her. It is known that solar radioactivity produces a low ambient level to which we are all exposed, and in regions where deposits of radioactive ores produce radon gas, we may be exposed to such gas which can accumulate in basements and closed spaces.
In nuclear research laboratories, on nuclear-powered ships, and in nuclear power plants, generally these are safe workplaces because of the scrupulous care in shielding design and in use of procedures which prevent exposure to radioactive sources. Occasionally, through accident or error, workers may be exposed to levels of radioactivity which may be dangerous. Since the personnel may not be aware of such exposure, it has long been standard procedure in such facilities to provide personal dosimeters for everyone to wear while at work. Each dosimeter is checked regularly to determine if the wearer has been exposed to a cumulative dosage which will require him to change jobs, or to undergo medical treatment.
Innovative forms of dosimeters include types which generate an alarm at a predetermined threshold, to warn the wearer to leave the area immediately. Regardless of their design, dosimeters require calibration to ensure that they remain accurate, since accuracy is synonomous with safety when dealing with the monitoring of radioactivity exposure.
Calibration of dosimeters was originally and is still largely done in laboratories which are remote from the facilities where the dosimeters are actually used. There has been a trend in recent years to calibrate the dosimeters on site, by using portable dosimeter calibrators. Such calibrators are typified by the structure in Jachter U.S. Pat. No. 3,107,299, the structure of which may briefly be summarized as follows. A holding jig, comprising a plurality of uniformly spaced retaining fixtures for dosimeters, is supported on a turntable driven by a precision motor controlled by a precision clock or clocks. A calibrated radiation source, enclosed within a shielded housing with a separately shielded, closable aperture, is located at a fixed position on the periphery of the turntable. In operation, the aperture shield is automatically moved aside during a timed exposure period, while the dosimeters are slowly moved past the aperture by the turntable, each dosimeter in turn facing the source at least once, and typically during several passes, to accumulate a known amount of radiation exposure. A hinged cover provides access to the interior for inserting and removing dosimeters. The shielding for the devices is comprised of layers, each layer being adapted to capture a different type of radiation.
A simple way of determining the dosage accumulated during a calibrating exposure period is to place in one of the retaining fixtures a reference dosimeter, recently calibrated by its manufacturer. The dosage recorded as accumulated on that reference dosimeter during the calibration period, can then be compared with the readings of each of the other dosimeters, to determine the appropriate calibration factor for each of those. This method is useful for all types of calibrators.
A major drawback of the design of the Jachter patent is that the radioactive source is located at the periphery of the turntable, and that its surrounding shielding makes its pattern of radiation quite directive. Therefore, the radiation it emits is most intense at the aperture of its shield and less intense at other points within the enclosure. This intensity depends on the point's distance from, and angle with respect to, the source. Dosimeters calibrators based on this design and its derivatives are relatively complex and costly.
Among the advantages of portable calibration units is that they could be used to do periodic calibrations when a dosimeter is turned in after an individual's work day, and thus to have the dosimeter ready for use the next day. When dosimeters are sent off-site for calibration, several days may elapse before they are returned. During that period, the worker must be provided with another dosimeter. When the original dosimeter returns, the radiation exposure during the intervening period, which was recorded on the spare dosimeter, must be entered in a logbook or data record for subsequent addition to the dosage accumulated on the primary dosimeter. Not only is such a process complex and susceptible to error, but also it is inappropriate for dosimeters which are designed to sound an alarm when a preset dosage threshold has been reached, since the dosimeter would have to be incremented by the dosage recorded on the spare dosimeter.
Alternatively, a spare dosimeter may be used to replace the primary dosimeter until the next calibration. This poses its own set of problems; it either doubles the number of dosimeters needed if each worker is assigned a primary unit and a spare, or, if a limited number of spares are used for the entire workforce, it necessitates a complex system of bookkeeping with a potential for error.
It is an object of the present invention to provide a novel portable dosimeter calibration device which may be used on-site at nuclear facilities.
Another object is to provide such a calibration device which is small, lightweight and relatively inexpensive to fabricate.
It is also an object to provide such a calibrator which is simple to use and which enables the user to avoid the complexities of record keeping, and the expense and inconvenience of sending dosimeters elsewhere for calibration.